1. Field of the Invention
The present invention generally relates to pulse shape modulation and an information recording and reproducing technique, and more specifically, to a pulse shape modulating technique for modulating a pulse shape when recording and reproducing information in and from a data recording medium using a pulsed laser beam.
2. Description of Related Art
Remarkable progress of personal computer functionality in recent years has allowed the personal computer to process audio-visual information, such as music and video images. Since the data volume of such audio-visual information is very large, optical disks, such as compact disks (CDs) and digital versatile disks (DVDs) have been attracting attention as large-volume information recording media. As the cost and price of such optical disks are reduced, optical disk apparatuses for recording and reproducing information in and from optical disks are spreading among users, and are increasingly used as components of computer peripheral equipment.
With an optical disk apparatus, a laser beam is guided onto the recording side of an optical disk, in which spiral or concentric tracks are formed, to form a fine spot on the disk in order to record and erase information in and from the disk. The information recorded in the optical disk is reproduced based on the light reflected from the recording side of the disk. In general, the optical disk apparatus is provided with an optical pickup device to emit a laser beam onto the recording side of the information recording medium and receive the reflected light from the recording side.
Generally, an optical pickup device includes a light source for emitting a laser beam at a prescribed light-emitting (output) power; a lens system for guiding the laser beam emitted from the light source onto the recording side of the information recording medium, as well as guiding the laser beam reflected from the recording side to a light receiving position; and a light receiving element arranged at the light receiving position.
In an optical disk, there are two regions provided, a mark (pit) region and a space (land) region having different reflectances from each other. Information is recorded on the optical disk by means of the respective lengths of the mark and the space and the combination thereof. The light emission power of the light source is adjusted so that the mark and the space are formed at predetermined locations in the optical disk during the recording operation.
For example, when a mark is formed in a recordable optical disk (hereinafter, referred to as a “dye-type disk” for convenience), such as CD-recordable (CD-R), DVD-recorddable (DVD-R), and DVD+recordable (DVD+R) having a recording layer containing an organic dye, the light emission power is raised to heat and melt the dye of the recording layer, thereby deforming or altering the nature of the substrate that is in contact with the melting portion of the recording layer. On the other hand, when forming a space, the light emission power is reduced as low as that in the reproducing operation, so as not to cause the substrate to deform or be altered in nature. As a result of the control of exposure power, the reflectance of the mark region is lower than that in the space region.
Examples of adjustment of the light-emission power are illustrated in FIGS. 1A through 1C. FIG. 1A illustrates adjustment of light emission power for a dye type CD, in accordance with writing data WD generated from recording data. The light emission power LP is raised to level Pw in order to form a mark in the disk, corresponding to the mark section M of the writing data WD. The light emission power is decreased to level Pr to form a space, corresponding to the space section S of the writing data WD. In this manner, the light emission power of the light source is regulated so as to pulse 4 light beam. The light emission power control shown in FIG. 1A is called single pulse recording control. Symbol Pw is referred to as peak power (or recording power), and Pr is referred to as bias power (or reproducing power). The peak power section of the light emission pulse is also referred to as a heating pulse, and the bias power section of the pulse is referred to as a cooling pulse.
FIG. 1B illustrates adjustment of light emission power for a dye type DVD, in which the light emission power produced to form a mark is divided into multiple pulses for the purpose of removing adverse effects of heat accumulation. Several heating pulses are given to form a single mark region. This type of light emission control is called multi-pulse recording control. Under multi-pulse recording control, the rule for rendering the light emission power into multiple pulses is called a recording strategy. Among the multiple pulses used to form a mark, the first heating pulse Pt is referred to as a leading heating pulse, and the last heating pulse Pf is referred to as a final heating pulse.
FIG. 1C illustrates adjustment of light emission power for rewritable disks with a special alloy contained in its recording layer (referred to as a “phase-change disk for convenience), including CD-rewritable (CD-RW), DVD-rewritable (DVD-RW), and DVD+rewritable (DVD+RW). When forming a mark in the rewritable disk, the special alloy is heated up to the first temperature using a laser beam, and then the light emission power is reduced for rapid cooling of the special alloy to make the special alloy amorphous. When forming a space, the special alloy is heated up to a second temperature that is lower than the first temperature and then slowly cooled to make the special alloy crystal. Accordingly, the reflectance of the mark region becomes less than that of the space region. In the example shown in FIG. 1C, multi-pulse recording control is employed for the purpose of removing the influence of heat accumulation. The light emission power Pe for forming the space is called erase power.
The above-described recording power and the recording strategy are the principal recording conditions that greatly affect the recording quality, and therefore, it is important to select the optimum recording power and the recording strategy for each optical disk in which data are to be recorded. JPA 11-66698 discloses a technique for selecting the optimum recording power. In this publication, several times of recording are conducted at various levels of recording power, and a power level that makes the jitter measured from the reproduced signal become minimum is selected as the optimum recording power.
However, when forming marks having the same length in writing data, the length of the actually formed marks may vary, depending on the lengths of the space located immediately before the mark (referred to as a “front space”) and the space located immediately after the mark (referred to as a “back space”). This is because radiant heat effect varies depending on the length of the space. Since the influence of heat accumulation in forming a mark also changes due to the recording power and the light emission time, the mark formed in the disk may not have the designed or expected length, depending on the size of the mark. This causes the quality of the reproduced signal to vary, and prevents accurate reproduction of information from the disk.